In the field of tachometry, two basic concepts have evolved. The first, and most conventional, involves the application of a tachogenerator producing a variable voltage alternating or direct current with voltage proportional to the speed at which the generator revolves. This system provides good accuracy over a wide speed range, but has the inherent disadvantage of possessing moving part which are subject to wear and require periodic maintenance. Additionally, if it is desired to measure the speed of machinery which operates at speeds of less than 5 revolutions per second, it is frequently necessary to overdrive the tachometer with respect to the machinery to be measured. This overdriving, or speed increasing mechanism involves the use of gears or belts and sheaves which themselves result in further complexity and inaccuracy and provide the additional requirement for efficient maintenance of the tachogenerator mechanism.
The second basic concept for measuring the speed of rotating or reciprocating machinery involves the use of means to detect the presence or absence of moving parts (for example, the teeth of a gear) and computes the speed of the parent machinery by generating a series of pulses of a fixed amplitude and duration by means of a monostable multivibrator (one pulse for each gear tooth, for example) then integrating the area of the pulses over a given period of time by means of a filter to produce a direct current analog signal proportional to the speed of the subject machinery. This same means is normally employed when the signal source is an alternating current tachogenerator, the frequency output of which is proportional to the speed of the machinery under measurement. The time constant of the integrating filter is a function of the frequency of pulses; that is, the lower the frequency of the AC signal or the repetition rate of the pulses, the longer must be the time constant of the filter in order to produce an analog signal reasonably free from ripple. This does not produce a problem at higher frequencies or pulse rates such as pulse rates of ten cycles per second or greater, but at low pulse rates such as pulse rates of less than two cycles per second, the time constant of the integrating filter produces serious lags in the response time for the system. For example, in response to a step change in input frequency of from 1 Hertz to 2 Hertz, a typical delay of ten to thirty seconds in response may be anticipated due to the long time constant of the integrating filter. This phenomena is often intolerable, and leads to the introduction of mechanical overdriving means to increase the input frequency of the tachometer. Obviously, the requirement for providing mechanical overdriving means introduces both electrical and mechanical disadvantages which may add materially to the cost of such system and which also introduces requirements for additional maintenance.